Astrophoto: Triple Star System Gliese 667 – Home of ‘Goldilocks’ Exoplanets

Here is a great new observation of the triple star system Gliese 667 from astrophotographer Efrain Morales of the Jaicoa Observatory in Puerto Rico. Recently, one of the stars, 667 C was found to have perhaps seven planets orbiting it! If all seven planets are confirmed, the system would consist of three habitable-zone super-Earths, two hot planets further in, and two cooler planets further out. Scientists say that the ‘f’ planet is “a prime candidate for habitability.”

Efrain also created an animation of the star system, showing the stars’ movements:

The animation was created using plates from the DSS (Digitized Sky Survey) with the final image in the animation from Efrain’s observations.

Gif animation of The animation of the triple star system Gliese 667, created using plates from the DSS (Digitized Sky Survey) with the final image in the animation from  Jaicoa Observatory. Credit and copyright: Efrain Morales. Click for animation.
Gif animation of The animation of the triple star system Gliese 667, created using plates from the DSS (Digitized Sky Survey) with the final image in the animation from Jaicoa Observatory. Credit and copyright: Efrain Morales. Click for animation.

The system is in the constellation of Scorpius and is just barely visible to the unaided eye at magnitude 5.9 – appearing as a single point of light. The three stars orbit each other in a complicated dance. The two brightest components of this system, Gliese 667 A and Gliese 667 B, are orbiting each other at about 13 times the separation of the Earth from the Sun, while Gliese 667 C is the smallest stellar component of this system, and orbits the other two stars at about 230 AU.

Efrain used a LX200ACF 12 in. OTA, F6.3, CGE mount, ST402xm CCD, Astronomik LRGB filter set.

Thanks to Efrain Morales and the Jaicoa Observatory for providing this latest look at an extremely interesting star system!

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Three Potentially Habitable Planets Found Orbiting Gliese 667C

A closer look at the previously-studied nearby star Gliese 667C has revealed a treasure trove of planets – at least six – with three super-Earths in the habitable zone around the star. Gliese 667C is part of a triple star system (Gliese 667) and is just over one third of the mass of our Sun. Now that we know there are multiple planets in the so-called Goldilocks zone – a region where liquid water could exist — Gliese 667C might be the best candidate for harboring habitable exo-worlds.

“We knew that the star had three planets from previous studies, so we wanted to see whether there were any more,” said Mikko Tuomi from the University of Hertfordshire in the UK, one of the astronomers who led the new study of Gliese 667C. “By adding some new observations and revisiting existing data we were able to confirm these three and confidently reveal several more. Finding three low-mass planets in the star’s habitable zone is very exciting!”

Artist’s conception of the seven planets possibly found orbiting Gliese 667C. Three of them (c, f and e) orbit within the habitable zone of the star. Image is courtesy of Rene Heller/ Carnegie Institution for Science.
Artist’s conception of the seven planets possibly found orbiting Gliese 667C. Three of them (c, f and e) orbit within the habitable zone of the star. Image is courtesy of Rene Heller/ Carnegie Institution for Science.

Tuomi, along with Guillem Anglada-Escudé of the University of Göttingen, Germany looked at existing radial velocity data from the HARPS spectrograph at ESO’s 3.6-metre telescope in Chile. The team said they are extremely confident on the data on the first five planets, while the sixth is tentative, and a potential seventh planet even more tentative.

The team writes in their paper:

Up to seven periodic signals are detected in the Doppler measurements of GJ 667C data, being the last (seventh) signal very close to our detection threshold.

The significance of the signals is not affected by correlations with activity indices and we could not identify any strong wavelength dependence with any of them.

The first six signals are strongly present in subsamples of the data. Only the seventh signal is unconfirmed using half of the data only. Our analysis indicates that any of the six stronger signals would had been robustly spotted with half the available data if each had been orbiting alone around the host star.

If all seven planets are confirmed, the system would consist of three habitable-zone super-Earths, two hot planets further in, and two cooler planets further out.

This diagram shows the system of planets around the star Gliese 667C. A record-breaking three planets in this system are super-Earths lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life. This is the first system found with a fully packed habitable zone. The relative approximate sizes of the planets and the parent star are shown to scale, but not their relative separations. Credit: ESO
This diagram shows the system of planets around the star Gliese 667C. A record-breaking three planets in this system are super-Earths lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life. This is the first system found with a fully packed habitable zone. The relative approximate sizes of the planets and the parent star are shown to scale, but not their relative separations. Credit: ESO

But the team said the three in the habitable zone are confirmed to be super-Earths. These are planets more massive than Earth, but less massive than planets like Uranus or Neptune. This is the first time that three such planets have been spotted orbiting in this zone in the same system.

“The number of potentially habitable planets in our galaxy is much greater if we can expect to find several of them around each low-mass star,” said co-author Rory Barnes from the University of Washington, “instead of looking at ten stars to look for a single potentially habitable planet, we now know we can look at just one star and find several of them.”

Gliese 667 (a.k.a GJ 667) is 22 light-years away from Earth in the constellation of Scorpius.
The planets in the habitable zone and those closer to the star are expected to always have the same side facing the star, so that their day and year will be the same lengths, with one side in perpetual sunshine and the other always night.

The researchers say that the ‘f’ planet is “a prime candidate for habitability.”

“It likely absorbs less energy than the Earth, and hence habitability requires more greenhouse gases, like CO2 or CH4,” the team wrote in their paper. “Therefore a habitable version of this planet has to have a thicker atmosphere than the Earth, and we can assume a relatively uniform surface temperature.”

The other stars in the triple system would provide a unique sunset: the two other suns would look like a pair of very bright stars visible in the daytime and at night they would provide as much illumination as the full Moon.

Are there more planets to be found in this abundant system? Perhaps, but not in the habitable zone. The team said the new planets completely fill up the habitable zone of Gliese 667C, as there are no more stable orbits in which a planet could exist at the right distance to it.

An artist’s impression of the orbits of the planets in the Gliese 667C system:

Read the team’s paper.

Sources: ESO, Carnegie , Planetary Habitability Laboratory

Potential ‘Goldilocks’ Planet Found

A new-found planet is in a ‘just-right’ location around its star where liquid water could possibly exist on the planet’s surface. A team of international astronomers have discovered a potentially habitable super-Earth orbiting a nearby star in a habitable zone, where it isn’t too hot or too cold for liquid water to exist. The planet, GJ 667Cc, has an orbital period of about 28 days and with a mass about 4.5 times that of the Earth. The star that it orbits is quite interesting. It is an M-class dwarf star and is a member of a triple star system and appears to be quite different from our Sun, relatively lacking in metallic elements.

The team said this discovery demonstrates that habitable planets could form in a greater variety of environments than previously believed.

“This was expected to be a rather unlikely star to host planets,” said Steven Vogt from UC Santa Cruz, one of the scientists involved in the discovery. “Yet there they are, around a very nearby, metal-poor example of the most common type of star in our galaxy. The detection of this planet, this nearby and this soon, implies that our galaxy must be teeming with billions of potentially habitable rocky planets.”

“This planet is the new best candidate to support liquid water and, perhaps, life as we know it,” said Guillem Anglada-Escudé, from the University of Gottingen in Germany. He was with the Carnegie Institute for Science when the planet was first discovered.

The planet orbits quite close to its parent star at 0.12 astronomical units, which is much closer than Mercury to the Sun. However, the Planetary Habitability Laboratory says the star is much dimmer and provides enough energy for the planet to possibly maintain similar terrestrial temperatures. There’s a caveat, though, that astronomers aren’t sure what the planet’s composition is, because they have not been able to measure its size; therefore, it could be a either a rocky or a gas planet. I would need to have a radius between about 1.7 and 2.2 Earth radii to be a rocky world.

The team used public data from the European Southern Observatory combined with observations from the Keck Observatory in Hawaii and the new Carnegie Planet Finder Spectrograph at the Magellan II Telescope in Chile. To follow up and verify the findings, the team used the radial velocity method to measures the small wobbles in the star’s motion caused by the gravitational tug of a planet.

“With the advent of a new generation of instruments, researchers will be able to survey many M dwarf stars for similar planets and eventually look for spectroscopic signatures of life in one of these worlds,” said Anglada-Escudé.

The star, GJ 667C is 22 light years away. It has much lower abundance of elements heavier than helium, such as iron, carbon, and silicon, as does our Sun. The other two stars (GJ 667A and B) are a pair of orange K dwarfs, with a concentration of heavy elements only 25% that of our Sun’s. Such elements are the building blocks of terrestrial planets so it was thought to be unusual for metal-depleted star systems to have an abundance of low mass planets.

Diagram of the planets orbiting the star GJ 667C. Credit: UC Santa Cruz

GJ 667C had previously been observed to have another super-Earth (GJ 667Cb) with a period of 7.2 days, although this finding was never published. This orbit is too tight, and thus hot, to support life. The new study started with the aim of obtaining the orbital parameters of this super-Earth, and came to find an additional planet.

The new planet receives 90% of the light that Earth receives. However, because most of its incoming light is in the infrared, a higher percentage of this incoming energy should be absorbed by the planet. When both these effects are taken into account, the planet is expected to absorb about the same amount of energy from its star that the Earth absorbs from the Sun. This would allow surface temperatures similar to Earth and perhaps liquid water, but this extreme cannot be confirmed without further information on the planet’s atmosphere.

The team said there is a possibility of other planets in the system, potentially a gas-giant planet and an additional super-Earth with an orbital period of 75 days. However, further observations are needed to confirm these two possibilities.

This is the fourth potentially habitiable extrasolar planet. Three were found in 2011: Gliese 581d, which scientists say is likely a rocky world about 20 ight-years away; HD 85512 b, another planet orbiting in a habitable zone is about 36 light-years away from Earth; and Kepler 22b, about 600 light-years away. Vogt was involved in the discovery of another planet in 2010 (Gliese 581g) in which he said the “chances of life on this planet are 100%,” but other astronomers have cast doubt on whether that planet even exists.

Papers:
The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample, and A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone (will add link when it becomes available)

Sources: UC Santa Cruz, Carnegie Institute for Science, Planetary Habitability Laboratory